I had some correspondence with the creator of the website. He said the tracks come from Youtube. Not sure how he manages it on a large scale, but he seems (falsely?) confident that he's protected by Youtube's fair-use laws because he isn't charging listeners.
Ever wanted to roll your own streaming music service? As it turns out, all it takes is $38 — and you don’t even need to touch a line of code.
For roughly the price of a Chromecast, you can buy a script that sets up a full-fledged Spotify-like streaming music app that pulls tunes from YouTube and lets users create accounts, discover popular albums and whip up playlists. Only, you wouldn’t be the first to launch such a service — and it may not be legal.